Create Spec.md

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+# Initial client spec
+
+## Structure
+
+Basic response format is a status object and a response object. The response can only be relied to be non-null when the overall status was successful (i.e 2xx)
+
+```json
+{
+  "status": {
+    "code": 200,
+    "message": "OK"    
+  },
+  "response": ...
+}
+```
+
+```json
+{
+  "status": {
+    "code": 504,
+    "message": "Timeout"    
+  }
+}
+```
+
+Addtionally addition context may be returned
+
+```json
+{
+  "status": {
+    "code": 200,
+    "message": "OK"    
+  },
+  "response": { "data": "here" },
+  "context": {
+    "logs": {
+      "stdout": "...",
+      "stderr": "..."
+    },
+    "container_exit_status": 0,
+    "invocation_info": {},
+    "timing": {}
+  }
+}
+```
+
+For errored requests, the status hash may include additional structured information about the error
+
+e.g. 
+
+```json
+{
+  "status": {
+    "code": 513,
+    "message": "Failure in container invocation",
+    "error": {...}    
+  },
+  "context": {
+    "logs": {
+      "stdout": "...",
+      "stderr": "..."
+    },
+    "container_exit_status": 0,
+    "invocation_info": {},
+    "timing": {}
+  }
+}
+```
+
+## Status codes 
+
+Codes have been chosen to follow HTTP style ranges but fall into 4 broad classes
+
+### Class I - platform routing errors
+
+The platform client (PipelineClient) is currently configured to retry on failure. Generally speaking, it should not retry on 4xx codes, and it's probably not woth retry for 501 and 502.
+
+503 and 504 can be retried with an appropriate backoff
+
+ - 500 generic platform error, ideally not used. 
+ - 501 unrecognised action (platform didn't recognise action type)
+ - 502 malformed request (message will contain detail)
+ - 503 no worker available (there is no worker - either generic or scoped - available to serve your request)
+ - 504 request timed out while waiting for a worker to complete
+
+### Class II - Platform worker errors
+
+Unless there is a deployment in progress, it is unlikely that any of these codes will benefit from retry. Two possible exceptions are 512 (which might be due to a recoverable network glitch) and 511 if (and only if) the targeted version is known to have been recently released.
+
+ - 510 generic worker error, ideally unused, but context will be provided in message and potentially in logging
+ - 511 container_version unavailable - the requested language:version pair isn't deployed. It may be in future
+ - 512 failure in container setup (container not spawned, maybe we're out of disk or s3 transfer failed)
+ - 513 failure in container invocation (container exited with non zero code)
+ - 514 output missing (container exited with a zero exit - so successfully - but the expected output was not written)
+
+
+### Class III - Worker Errors (potentially) derived from user input
+
+It is expected that all 4xx error codes are context independent. Repeated submissions will continue to error in the same way, so retrying is pointless.
+
+ - 400 bad input (generic, as yet not used - but we could preliminary scan for vulnerabilities for instance)
+ - 401 forced exit due to timeout
+ - 402 forced exit due to excessive IO
+ - 403 forced exit - other
+
+### Class IV - successful container run
+
+ - 200 ok, successful invocation. There is a value in 'response', it may indicate a successful or failed or errored test run, but the platform considers the test_runner as having completed its work correctly. Data in response is ready for returning to upstream systems.